1. Field of the Invention
The present invention relates to methods of controlling charge and discharge of non-aqueous electrolyte secondary cells such as lithium secondary cells.
2. Description of Related Art
A cell that has in recent years drawn attention as having a high energy density is a non-aqueous electrolyte secondary cell in which the negative electrode active material is composed of metallic lithium, an alloy or carbon material that is capable of intercalating and deintercalating lithium ions and the positive electrode active material is composed of a lithium-transition metal complex oxide represented by the chemical formula LiMO2 (where M is a transition metal).
A representative example of the lithium-transition metal complex oxide is lithium cobalt oxide (LiCoO2), which has already been in commercial use as a positive electrode active material for non-aqueous electrolyte secondary cells.
Materials containing Mn as a transition metal and those containing Ni as a transition metal have also been researched, and materials containing all of Mn, Ni, and Co have been actively researched. (See, for example, Japanese Patent Nos. 2561556 and 3244314; and Journal of Power Sources, 90 (2000) pp. 176-181).
It has also been reported in Electrochemical and Solid-State Letters, 4(12) A200-A203 (2001), for example, that among the lithium-transition metal complex oxides containing Mn, Ni, and Co, a material represented by the chemical formula LiMnxNixCo(1-2x)O2, in which the contents of Mn and Ni are equal, shows exceptionally high thermal stability even in a charged state (high oxidation state).
Japanese Unexamined Patent Publication No. 2002-42813 reports that a complex oxide containing Ni and Mn at substantially equal amounts shows a voltage of about 4 V, which compares with LiCoO2, and exhibits high capacity and good charge-discharge efficiency.
A cell employing a positive electrode using as its main material (50 weight % or more) such a lithium-transition metal complex oxide containing Mn, Ni, and Co and having a layered structure (for example, the one represented by the chemical formula LiaMnbNibCo(1-2b)O2 (0≦a≦1.2, 0<b≦0.5)) is expected to improve cell reliability remarkably because such a cell has high thermal stability during charge.
Nevertheless, through a study on cell charge-discharge performance, it was found that a cell in which the positive electrode active material is such a lithium-transition metal complex oxide containing Mn, Ni, and Co and having a layered structure and the negative electrode is made of a material having a higher initial charge-discharge efficiency than that of the positive electrode active material, such as graphite, shows a significantly inferior performance to a conventionally-used cell adopting lithium cobalt oxide for the positive electrode.
As for the charge-discharge conditions for non-aqueous electrolyte secondary cells using the above-noted lithium-transition metal complex oxide as a positive electrode active material and a carbon material as a negative electrode active material, Japanese Unexamined Patent Publication No. 2002-42813, for example, discloses that the end-of-charge voltage should be 4.2 V and the end-of-discharge voltage should be 2.5 V.
Also, Japanese Unexamined Patent Publication No. 2003-17052 discloses that the end-of-charge voltage should be 4.2 V and the end-of-discharge voltage should be 2.0 V. Further, Japanese Unexamined Patent Publication No. 7(1995)-153494 describes in paragraph [0015] that generally the end-of-discharge voltage should be 2.0 V for a non-aqueous electrolyte secondary cell adopting a similar complex oxide containing Ni and Co as the positive electrode active material and a carbon material for the negative electrode. From these descriptions, it is thought that the end-of-discharge voltage for cells that use a lithium-transition metal complex oxide containing Ni as the positive electrode and a carbon material as the negative electrode is about 2.0-2.5 V.
To date, as an example of a method for improving the charge-discharge cycle performance of a non-aqueous electrolyte secondary cell that uses as the positive electrode active material a complex oxide containing Li, Ni, and Co and having a layered structure, Japanese Unexamined Patent Publication No. 5(1993)-290890, for example, has proposed that the capacity ratio of the positive electrode and the negative electrode during charge should be set so that the range of x in LixMO2 (where M is Co and/or Ni) of the positive electrode falls within the range 0.35≦x≦0.9. (Note that the value x decreases during charge and increases during discharge.) Paragraph [0009] of Japanese Unexamined Patent Publication No. 5(1993)-290890 describes that in this case, the use of LixMO2 having such a composition range can suppress destruction of the structure of the positive electrode active material resulting from charge/discharge cycles.
Nevertheless, it has been found that cells that use the lithium-transition metal complex oxide containing Mn, Ni, and Co as the positive electrode active material show remarkably poor charge-discharge cycle performance although the value x (lithium content) during charge and discharge falls within the range specified in Japanese Unexamined Patent Publication No. 5(1993)-290890, and this method is incapable of attaining sufficient advantageous effects.
Further, Japanese Unexamined Patent Publication No. 7(1995)-153494 discloses that although using a complex oxide containing Ni and Co and having a layered structure as a positive electrode active material, an increase in the potential of the negative electrode in the final stage of discharge of a cell is suppressed by reducing the initial charge-discharge efficiency of the positive electrode to be lower than the initial charge-discharge efficiency of the negative electrode, thereby suppressing decomposition of the electrolyte solution on the negative electrode surface. However, with the above-described cell that uses a lithium-transition metal complex oxide containing Mn, Ni, and Co as a positive electrode active material, the initial charge-discharge efficiency of its positive electrode is lower than the initial charge-discharge efficiency of the graphite negative electrode. Therefore, although the potential of the negative electrode does not increase in the final stage of discharge of the cell, its charge-discharge cycle performance is remarkably poor, and it has been found that a sufficient effect cannot be attained even when degradation in the negative electrode side is suppressed.